Coated abrasives, considered the premier tool for abrading and finishing wood and wood-like materials, unfortunately suffer from two major problems associated with their use, loading and static electricity. Loading is the term used to describe the filling of the spaces between abrasive granules with swarf (the material removed from the workpiece) and the subsequent build-up of that material. In wood sanding, sawdust particles are lodged in between abrasive grains, dramatically reducing the cutting ability of the grains and often resulting in burning of the surface of the wood workpiece. Burning, of course, severely damages the workpiece.
A major cause of loading is the build-up of an unwanted electrical charge during belt sanding. This electrical charge or static electricity is generated by the constant interaction of the endless coated abrasive belt, the wood workpiece and the back support for the endless belt. This static charge is typically on the order of 50 to 100 kilovolts.
Static electricity is also responsible for numerous other problems. A sudden discharge of the accumulated static charge can cause serious injury to an operator and ignition of dust particles, thereby posing a serious threat of fire or explosion. The static charge also causes the sawdust to cling to various surfaces including that of the coated abrasive and the wood workpiece thereby making it difficult to remove by use of a conventional exhaust system.
If the electrical charge is reduced or eliminated, the coated abrasive will have a significantly longer use life, produce a finer surface finish on the workpiece and eliminate or reduce the potential for the above-mentioned hazards.
Many solutions, with varying degrees of success, have been attempted to solve the loading and static electricity problems. One solution is to apply a size or supersize coat of low energy surface material onto the abrasive surface. U.S. Pat. No. 2,768,886 (Twombly) discloses the use of an unfused surface coating of a metal soap such as a metal stearate or palmitate on a coated abrasive to reduce the loading. U.S. Pat. No. 2,881,064 (Rankin et al) discloses the use of an organo silicon based supersize coating to prevent loading. U.S. Pat. No. 3,042,508 (Hagis et al) discloses a metal backed-abrader having a superposed coating of fluoroethylene polymer to reduce loading. U.S. Pat. No, 3,043,673 (Klein et al) teaches the use of an oxy-containing compound selected from aliphatic polyhydric alcohols or aliphatic polyethers blended into the phenolic based size resin. U.S. Pat. No. 3,795,496 (Greenwood) describes a coated abrasive containing plasticized polyvinyl acetate supersize coating to prevent loading. U.S. Pat. No. 4,396,403 (Ibrahim) describes a loading resistant coated abrasive made without the need of a supersize coat having a size coat comprised of a mixture of phosphoric acids, partial esters of such acids, amine salts of such acids and partial esters, and/or quarternary ammonium salts with at least one long substituent group and amino resins or glue sizing resin.
Another solution to the loading problem is to incorporate a conductive or an antistatic material into the coated abrasive construction to eliminate the accumulation of electrical charge. U.S. Pat. No. 3,942,959 (Markoo et al) discloses a coated abrasive construction having an inner conductive layer which has been made conductive by the addition of conductive filler sandwiched between two nonconductive resin layers to prevent the accumulation of electrostatic charge during grinding. The conductive filler may be a metal alloy, metal pigment, metal salt or metal complex. U.S. Pat. No. 3,992,178 (Markoo et al) teaches a coated abrasive containing an outer layer containing graphite particles in a bonding resin which reduces the electrostatic charges generated during grinding.
Although the above references may produce coated abrasive products which have a lessened tendency to load, there is still considerable room for improvement.